It is well known that extremely high voltages can develop in the vicinity of an integrated circuit due to the build-up of static charge. A high potential may be generated to an input or output buffer of an integrated circuit, which may occur by a person simply touching a package pin, which is in electrical contact with an input or output buffer of the integrated circuit. When the electrostatic is discharged, a high current is produced at the package nodes of the integrated circuit. ElectroStatic Discharge (ESD) is a serious problem for semiconductor devices since it has the potential to destroy the entire integrated circuit.
The duration of the ESD transient is very short, typically in the order of nanoseconds, so that the conventional circuit breakers do not react quickly enough to provide adequate protection. For this reason, it is rapidly becoming a known practice to incorporate an ESD protection system in the circuitry itself. Conventionally, bi-directional diode strings are coupled between the nodes to protect the circuit. Other ESD devices such as transistors are also being used. ESD devices are also widely used between power lines to protect the internal circuit coupled between power lines and to discharge the ESD current to the ground.
For the purpose of decoupling noise, power lines in the integrated circuit are separated so that the noise generated in one part of the circuit does not affect another part of the circuit. However, the separation of the power lines creates more power lines that are subject to ESD. Therefore, more power line ESD protection schemes were developed.
FIG. 1 illustrates a conventional power domain ESD protection circuit. VDD power line 2 and VSS power line 6 form one power domain and VDD power line 4 and VSS power line 8 form another power domain. Internal circuit 10 is coupled between power lines 2 and 6 and internal circuit 12 is coupled between power lines 4 and 8. Bi-directional diode string 24 is used to protect circuit 10, and bi-directional diode string 30 is used to protect circuit 12. Since ESD may occur between power lines of different power domains, such as power lines 2 and 4, 6 and 8, ESD protection circuits are also required between power domains. As shown in FIG. 1, bi-directional diode string 14 is coupled between power lines 2 and 4, and a bi-directional diode string 18 is coupled between power lines 6 and 8. If a voltage transient occurs between power lines 2 and 4, the diode string 14 conducts in one direction and provides a path for electrostatic charges. Similarly, the diode string 18 between power lines 6 and 8 helps to eliminate the voltage transient between these two power lines.
When more power domains are used in the integrated circuit, every two adjacent power lines should be protected by an ESD protection circuit, therefore, the number of protection circuits increases dramatically. A more effective protection scheme, which was taken from U.S. Pat. No. 6,144,542, is devised by using ESD buses, as shown in FIG. 2. ESD buses 102, 108 and 112 formed of highly conductive materials are added into the circuit. Each VDD power line is connected to an ESD bus through an ESD connection cell wherein the ESD connection cells 11, 21, 31, . . . k1 are normally bi-directional diode strings or devices made of silicon-controlled rectifiers (SCR). VSS power lines are connected to an ESD bus 102 through ESD connection cells 12, 22, 32, . . . k2. The ESD buses are coupled with ESD clamp devices 100, 104 and 106, wherein the ESD clamp devices can be made of bi-directional diode strings or SCRs.
Comparing to the prior art in FIG. 1, instead of being coupled to each adjacent power line through an ESD protection device, a power line only connects to one ESD bus through an ESD connection cell. This greatly reduces the number of ESD protection devices required. In an example with a typical scenario, a transient occurs at one node in internal circuit i and a grounded node is at a node in another internal circuit j, where i and j are circuit numbers. The ESD current needs to go through at most two ESD connection cells, one connecting circuit i and the ESD bus and one connecting circuit j and the ESD bus. The ESD current also goes through at most one ESD clamp device in order to be conducted from one ESD bus to another.